This study was initiated to determine the structure:function relationship of RNA polymerase II of Drosophila melanogaster. This enzyme is a heteromultimer consisting of approximately ten different subunits, each of which is presumably specified by a different locus. To date one locus, a mutant (C4) of which confers alpha-amanitin resistance, has been identified. We plan to clone C4 to determine which subunit of the enzyme is specified by C4. The cloned segment of the C4 locus will be used to study the molecular organization of that section of Drosophila X chromosome to see if genes coding for other subunits of RNA polymerase II reside there. To broaden our scope of understanding of transcription by RNA polymerase II, we also plan to use this system to identify non-polymerase II components of the transcription apparatus.